Study On Micro-deformation Behaviors And Cutting Simulation Of Dual-phase Titanium Alloy At Grain Scale

With the improvement of industrial production and the social needs,the high integration and miniaturization of products have developed in many fields,such as national defense,communications and medicine.Micro-cutting is widely used for manufacturing micro parts because of its fast,efficient,low-cost,wide application range and the ability to process complex three-dimensional geometric structures.Among the many micro-components,micro-electromechanical systems such as micro-robots and micro-aircraft are emerging fields developed in many countries.Micro-power devices are the "heart" of micro electro mechanical systems,so the micro impellers or turbines are very important.Because they work under harsh conditions such as high temperature,high pressure and alternating load,they are usually produced by the materials with low density,high specific strength and high temperature resistance,such as Ti6A14V.Ti6A14V is a dual-phase polycrystalline material composed of α phase with HCP lattice structure and β phase with BCC lattice structure,so the coordinated deformation and interaction mechanism of the dual phases during plastic deformation are complex.In addition,the grain features such as orientation,grain boundary and phase boundary will significantly affect the machinability of the workpiece and the quality of the product since the micromachining size is approximately equal to the grain size in microns.At present,most of the constitutive theories for micromachining are to improve the traditional Johnson-Cook constitutive,which cannot directly reflect the influence of grain information on machinability.In this paper,a dual-phase crystal plasticity finite element model suitable for Ti6Al4V was established by crystal plasticity theory to explore its micro deformation behaviors under dynamic and static conditions.Further,the microcutting simulation model was established to study its machinability.Firstly,the actually dual-phase grain size information of Ti6A14V was extracted from EBSD data,then the distribution law of dual-phase grain size information was obtained.The generated information of grain size,orientation and grain boundary misorientation was obtained pseudorandomly according to the distribution law.Then,the dual-phase geometric model was established by the pseudorandom grain sizes and Voronoi method.Further,the dual-phase microstructure model was established by assigning pseudorandom orientations to the dual-phase geometric model.Secondly,the constitutive model was calibrated by quasi-static tensile experiments to obtain the constitutive parameters of dual phases.The model was verified by experiments at the micro and macro levels.The micro level means comparing the grain sizes after stretching in the X direction between the simulation and experiment.The macro level means comparing the stress-strain curves between the Y-direction model and experiment.Thirdly,four situations were introduced to study the effects of dual-phase morphology distribution on mechanical behaviors of Ti6A14V,including Model 1(dual-phase grains are randomly distributed),Model 2(dual-phase grains are homogeneously distributed),Model 3(dual-phase grains are periodically distributed perpendicular to the tensile direction)and Model 4(dual-phase grains are periodically distributed parallel to the tensile direction).The results showed that the Model 4 is the best in the comprehensive mechanical properties during quasi-static tensile process because of high tensile strength and good micro stress-strain uniformity.Then,the split hopkinson pressure bar experiments were carried out to fit the crystal plasticity constitutive parameters under dynamic conditions because the quasi-static constitutive is not suitable.Then,the stress-strain distributions of the dual-phase under dynamic conditions were quantitatively analyzed by the dynamic model.The results showed that the strain distribution of the material tends to be uneven with the increased strain rate at high strain rate situations,which easily leads to crack initiation,extension and failure of the material.Compared with the a phase,the flow stress of the β-phase grains is more sensitive to the strain rate.The flow stress of the β-phase grains is more sensitive to the strain rate compared with the a phase.Finally,the fracture model was added to established the microcutting model for Ti6A14V.The model was verified in terms of cutting force and surface roughness by orthogonal cutting experiment with diamond cutter.Then,the machinability of dual-phase grains at different cutting speeds was investigated,including cutting force and surface roughness.The results showed that phase boundary is more cutting resistant than grain boundary.The average cutting force of α-phase grains is higher than that of β-phase grains.The overall β-phase grains are significantly more sensitive to cutting speed than α-phase grains in terms of cutting force.At different cutting speeds,the surface roughness value of β-phase grains is significantly larger than that of α-phase grains,and the surface roughness value of β-phase grains is more sensitive to cutting speed.